Vibration isolator

ABSTRACT

A vibration isolator suspension element for use in a vibration isolating system. The suspension element comprises first and second sets of parallel, spaced-apart U-springs which longitudinally extend at right angles and in parallel planes to one another. Means are provided for connecting the sets of Usprings in series relation as well as means for mounting the element to both a supporting structure and to a mass to be supported. The element is also provided with integral means for providing friction damping during both translational and rotational motion to which the mass may be subjected.

nited States Patent Inventors Philip Marshall Lexington; Joseph C.Boeggeman, Beverly, both of Mass. Appl. No. 65,592 Filed Aug. 20, 1970Patented Dec. 7, 1971 Assignee Marshall Research 8: Development Corp.

Burlington, Mass.

VIBRATION ISOLATOR 12 Claims, 7 Drawing Figs.

US. Cl 248/358 R, 188/1 B, 248/20, 267/158 Int. Cl Fl6f15/04 Field ofSearch 248/358 R,

[56] References Cited UNITED STATES PATENTS 2,359,941 10/1944 Rosenzweig248/21 3,057,592 10/1962 Thrasher 248/358 3,066,905 12/1962 Gertel248/358 3,151,833 10/1964 Thrasher... 248/358 3,565,386 2/1971 Lemkuil248/358 Primary Examiner-J. Franklin Foss Attorney-lrving S. RappaportABSTRACT: A vibration isolator suspension element for use in a vibrationisolating system. The suspension element comprises first and second setsof parallel, spaced-apart U-springs which longitudinally extend at rightangles and in parallel planes to one another. Means are provided forconnecting the sets of U-springs in series relation as well as means formounting the element to both a supporting structure and to a mass to besupported. The element is also provided with integral means forproviding friction damping during both translational and rotationalmotion to which the mass may be subjected.

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VIBRATION ISOLA'IOR This invention relates to vibration isolationsystems generally and more particularly, to a novel and improvedvibration isolator with an integrally formed friction damper.

Vibration isolators are used to reduce the magnitude of vibratory forcestransmitted from one member or element to another member or element. Itmay be desired to reduce the magnitude of forces transmitted from asupported element to its supporting structure or vice versa. While notnecessarily limited thereto, this invention is concerned with reducingthe magnitude of vibration forces transmitted from a supportingstructure to a supported device.

In such prior art devices as characterized by US. Pat. No. 3,066,905vibration isolation in the form of friction damping, has been providedfor the supported device with respect to the usual three coordinates oftranslational vibration. However, no such vibration isolation in theform of friction damping has been provided for the three coordinates ofrotational or angular vibration. In US. Pat. No. 3,066,905 the frictiondampers are separate thrust-type units not integrally formed with thewhole isolator unit and are focused at the elastic center of the system.By focusing these thrust-type friction dampers at the elastic center ofthe system, friction damping for translational, but not for rotational,motion is obtained. Since the dampers are focused at the elastic centerfor torsional or rotational motion where the moment arm between thefriction force and the axis of rotation is zero, there is no frictionmoment and therefore no frictional damping during rotational motion.Friction damping during rotation is not provided in US. Pat. No.3,066,905 because of the rotational hysteresis created in the rotationalmotion of a guidance system or stable platform holding a gyro withrespect to the mounting base. This hysteresis may result in an angularlag across the isolator unit between the base and the isolated platform.However, we have discovered that under vibration this angular lagdisappears very rapidly regardless of the application of frictiondamping during rotational motion. Therefore, the present inventionprovides frictional damping for both translational and rotationalmotion.

Further, the thrust-type friction dampers disclosed in [1.8. Pat. No.3,066,905 are the type which use a spring wrapped around the unit. Inorder to change the friction setting the spring must be removed andphysically shortened or lengthened appropriately so as to apply thecorrect pressure to obtain the desired frictional damping. Anotherdisadvantage of these thrust-type friction dampers is that they are notintegral with the isolator unit, thereby requiring more space andresulting in a unit of greater size and weight. This is a severedisadvantage especially in aircraft and missile applications where sizeand weight considerations are of paramount importance. Also, in theprior art unit the U- or C-springs of the isolator are riveted to theplates so that the spring rate of each spring is not readilycontrollable because the damping force of the rivets is highly variable.

It is an object of the present invention to provide a new and improvedvibration isolator assembly for isolating and damping a supported devicefrom the three coordinates of translational motion and from the threecoordinates of rotational motion.

Another object of the present invention is to provide a vibrationisolator assembly in which friction dampers are integrally formed withthe isolator assembly.

Still another object of the present invention is to provide a vibrationisolator with integral friction damping which eliminates extraneousbrackets and support surfaces resulting in a more compact and lighterweight unit.

Yet another object of the present invention is to provide a vibrationisolator system which is balanced and fully decoupled.

A further object of the present invention is to provide a vibrationisolator assembly using springs to provide vibration isolation in whichthe spring rate of each spring may be accurately controlled.

The above objects advantages and features of the present invention, aswell as others, are accomplished by providing a vibration isolatorsuspension element for use in a vibration isolating system comprising: afirst set of parallel spaced-apart U-springs; a second set of parallelspaced-apart U-springs longitudinally extending at right angles to thefirst set of U-springs and in general plane extending parallel to thegeneral plane of the first set of U-springs; means for connecting saidfirst and second set of U-springs in series relation; means for mountingone end of said suspension element on a supporting structure; means formounting on the other end of said suspensionelement a mass to besupported; and means provided integrally with said suspension elementfor providing friction damping during both translational and rotationalmotion to which said supported mass may be subjected.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter. The invention accordingly comprises theapparatus possessing the construction, combination of elements, andarrangement of parts which are amplifred in the following detaileddisclosure and the scope of the application which will be indicated inthe claims.

For a fuller understanding of the nature and objects of the presentinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings herein:

FIG. 1 is a partial cross-sectional view of a vibration isolation systememploying the vibration isolator of the present invention;

FIG. 2 is an isometric view of a vibration isolator of the presentinvention;

FIG. 3 is a top plan view of the vibrator isolator shown in FIG. 2;

FIG. 4 is a bottom view of the isolator shown in FIG. 2;

FIG. 5 is a side elevational, cross-sectional view isolator the isolatorshown in FIG. 3 taken along the line 5-5;

FIG. 6 is an enlarged, partial cross-sectional view of a portion of theisolator shown in FIG. 2; and

FIG. 7 is a blowup view of a cross section of a portion of the isolatoras shown in FIG. 5.

FIG. I shows one example of an application of the vibration isolator ofthe present invention. In this application of the vibration isolator 10of the present invention, typically a base or structure 11 as seen inFIG. 1 for supporting and isolating a mass 13, such as a navigationalinstrument or device, has mounted thereon a plurality of angularlyspaced-apart brackets such as illustrated by a bracket 12. Each brackethas mounted thereon a spring suspension element generally indicated at14. The spring suspension elements are each connected to a supporting ormounting ring or member 15 which is adapted to support the mass to beisolated. This mass 13 may be a navigational instrument or the like. Thespring suspension elements provide the sole support of the mass beingisolated and also provide the isolation of the mass from vibration.Friction damping is provided during both translational and rotationalmotion by a plurality of friction dampers one of which is indicatedgenerally at 116 in FIG. 3. Friction dampers 16 are formed integrallywith the spring suspension element 114 and as a part of the overallisolator unit 10.

In understanding the present invention, the nature and construction ofthe spring suspension elements M should be con sidered. Each of thesesuspension elements comprises a plurality of supporting and isolatingelements each of which is U- shaped and which will be hereinafterreferred to as U-springs. By a U-spring is meant a generallychannel-shaped member having a resiliently deformable web portion whichin cross section is redirected in the sense of a U-shape, cross section,or in the sense of cross sections such as are often referred to assinuous or corrugated cross sections. The characterizing fea ture of aU-spring, for the purpose of this invention, is that it is resilientlyflexible in a cross-sectional plane both in a first direction, whereinthe web portion tends to be compressed or elongated, and a seconddirection extending at right angles to the first direction, wherein theweb is skewed so that the opposite ends of the web tend to be laterallyoffset from each other, and is at least substantially stiffer, if notsubstantially rigid, in the direction extending at right angles to thecrosssectional plane of the U-spring. In each suspension element thereare at least two U-springs which are angularly offset so that thecross-sectional plane of one U-spring extends at right angles to thecross-sectional plane of the other U-spring. One of the ends of each ofthe U-springs is rigidly connected to one of the ends of the otherU-spring so that the two springs are arranged in series relation. Theother end of one of the U-springs is rigidly connected to means formounting the mass to be supported and isolated, and this end of theseries arranged U- springs in each of the angularly spaced-apartsuspension elements is rigidly connected to the corresponding ends ofthe other series arranged U-springs in the remaining suspensionelements. The other end of the other U-spring in each series arrangedpair thereof is rigidly connected to a common base or supportingstructure.

The U-springs or suspension elements provide the sole resilient supportof the mass being isolated as well as providing the vibratory isolationof the mass. The friction dampers provide the desired friction dampingduring both translational and rotational motion of the mass. Thesuspension elements and friction dampers are preferably arrangedrelative to the mass being supported to provide a fully decoupledisolation system. By a fully decoupled isolation system is meant avibratory isolation system (1) wherein the elastic center of the systemcoincides with the center of gravity of the mass being supported (thisdecouples the spring suspension elements), and (2) wherein theapplication of motion in any direction generates friction forces ormoments only in the direction of the applied motion and not in thedirection of the other five coordinates of motion (this decouples thefriction dampers). By the term elastic center is meant the point in asystem of spring supports to which a linear force may be applied fromany direction with a resulting purely translational movement of the masssupporting structure of the system. Thus, in a decoupled isolationsystem, a linear force exerted on the center of gravity of the supportedmass from any direction will result a pure translational movement of themass with no angular component of movement. When a U-spring isolationsystem of the type described is used to support a mass such as anavigational device having an azimuth axis, the suspension elements arepreferably arranged relative to the azimuth axis of the system and thecenter of gravity of the mass being supported and are constructed toprovide a decoupled system. While linear forces on the supported devicewill tend to result in pure translational movement of the device, arotational input force applied to the device will, of course, tend toprovide angular movement of the device either about its azimuth axis orabout an axis extending at right angles thereto, which will be referredto as a tilt axis, The tilt axis may correspond to one of the axesgenerally referred to as the pitch and roll axes, or an axis spacedangularly between the pitch and roll axes. By arranging the suspensionelements in a predetermined relation with respect to the azimuth axis,so as to arrange the series connected U-springs in either parallel orperpendicular relation with respect to the azimuth axis or inclinedrelative to the azimuth axis, the system may be provided withsignificantly increased stiffness with respect to movement of the deviceabout a selected one of the azimuth or tilt axes or with respect to boththe azimuth and a tilt axis. This increased stiffness may beparticularly desirable in the case of a navigational device, where it isdesired to provide as little movement of the device as possible abouteither or both the azimuth and/or the tilt axes. In the case of anavigational device, it may also be desirable to assure that followingangular displacement of the device about either the azimuth axis or atilt axis the device will be returned by the suspension elements to itsinitial position with a high degree of accuracy.

It is necessary, of course, that the U-springs provide sufficientstiffness to provide a satisfactory resilient support of the device.Also, it may be desirable to provide for displacements of the mass whichare relatively large as compared to the stiffness of the U-spring. Also,it is desirable to provide a U-spring of relatively small dimensions inorder to conserve space. Further, it is desirable to provide frictiondamping in the system, and in this connection, and in the interest ofeconomy and simplicity of construction, the friction dampers are formedintegrally with the overall isolator unit. In order to provide aU-spring with the combined features of relatively high stiffness, theability to operate with relatively large vibratory displacements, andsmall size, the U-springs are provided in a laminated configuration,wherein each spring comprises a plurality of nested U-spring members. inorder to obtain the accurate retumability desired, at least thedeformable portions of the U-spring members making up each laminated U-spring are slightly spaced apart to eliminate rubbing contact betweenthe deformable portions of the U-spring members during deformation ofthe' suspension elements. In this manner, any hysteresis effect due tointerfacial friction forces resulting from rubbing contact of theU-spring members is eliminated, and a higher degree of angularretumability of the system is provided. The friction dampers areutilized to provide system damping, and need not be focused at theelastic center of the system since we have discovered that any angularlag created by rotational hysteresis disappears very rapidly regardlessof the application of friction damping during rotational motion. Sincethe friction dampers are not focused at the elastic center, the vectorsrepresenting the friction forces do not pass through the elastic center.The only requirement as far as the positioning of the friction dampersis concerned is that they must be balanced with respect to the elasticaxis. By balanced" it is not meant that the friction forces applied bythe dampers need be equal in all directions but that the application ofmotion in any direction generates friction forces or moments only in thedirection of applied motion and not in the direction of the other fivecoordinates of motion. Under this definition, there are a number of waysof positioning the friction dampers in the present invention.

Referring now to FIGS. 2-7, bracket 12 is an L-shaped member having avertical portion 18 and a horizontal portion 20. Vertical portion 18 hasan opening 22 passing therethrough near its uppermost end. Horizontalportion 20 has two openings 24 and 26 at opposite sides thereof whichpass completely therethrough. Openings 22, 24, and 26 are provided formounting bracket 12 to the base or support platform (not shown) for themass which it is desired to support. Vertical portion 18 also has twosubstantially rectangularshaped openings 28 and 30 passing therethrough.Openings 28 and 30 permit two of the U-springs of spring suspensionelement 14 to be positioned and mounted properly with respect to bracket12. Vertical portion 18 also has a center opening 32 in the form of ahole passing therethrough for the purpose of allowing proper operationof the spring suspension element 14 and support of the mass to besupported as will be described.

To provide the necessary support for the vertical and horizontalportions 18 and 20 of bracket 12, integral side walls 34 and 36 areformed at approximately a 45 angle between portions 18 and 20. Verticalportion 18 also has a pair of flanges 38 and 40 formed adjacentsidewalls 34 and 36, respectively. Sidewalls 34 and 36 and theirrespective corresponding flanges 38 and 40 have opening 42 and 44passing therethrough. Openings 42 and 44 permit access to the frictiondampers 16 for adjustment thereof as will be described.

The suspension element 14 comprises a plurality of stacked platelikemembers including the portion of bracket 12 between openings 28 and 30,centerplate 46 and top plate 48. The portion of bracket 12 betweenopenings 28 and 30 and centerplate 46 are resiliently connected by apair of elongated, spaced-apart U-springs 50 and 52. The bight or webportion 54 of each spring 50 and 52 is shown as being disposed outwardlyof the longitudinal edges of centerplate 46 and through opening 28 and30 in bracket 12. However, it is possible, if desired, to dispose thebight portion of the springs between the plates and inside thelongitudinal edges thereof.

Springs 56 and 52 are each fixed to the portion of bracket 112 betweenopenings 28 and 30 by screws 60 and threaded cylindrical inserts or nutssuch as shown at 62. Screws 60 pass through backup strips 64, throughsprings 50 and 52 and through bracket I2. Nuts 62 (not shown) which restin recesses in the surface of bracket I2 are secured to screws 66 tofirmly fix springs 58 and 52 to bracket I2. The heads of screws 60 arerecessed in backup strips 64 so that their heads lie flush with thesurface of strips 64. Nuts 62, when in place, lie flush with the surfaceof the inside wall of bracket 12. Similarly, springs 58 and 52 aresecured to centerplate 46 by screws 60 and nuts 62 and backup strips 64.With the springs held at one, their ends between bracket 12 and backupstrips. 64 and at the other of their ends between plate 46 and backupstrips 64, a very firm and secure connection is thus made. By employingas fasteners screws 60 and nuts 62, instead of rivets, the clampingaction applied to the ends of the springs to provide good end fixity ofthe springs can be accurately controlled. Each screw 60 may have ameasured torque applied thereto so as to provide an exact and uniformend fixity at each end of springs 50 and 52.

With the connection of the portion of bracket 12 between openings 28 and30 and centerplate 46 at their longitudinal edges by oppositely facingU-springs 50 and 52, it will be apparent that with bracket 12 fixed tothe base or support platform centerplate 46 will be permitted movementtoward and away from bracket I2 wherein the connecting web or bightportion 54 of U-springs 50 and 52 will tend to be compressed orelongated. More specifically, during movement of the plate 46 toward thebracket 12, the cross-sectional configuration of the bight 54 ofU-springs 50 and 52 will be deformed from the generally semicylindricalconfiguration shown in FIG. 3 to a generally semiellipsoidconfiguration. Also, centerplate 46 will be permitted movement in adirection parallel to the general plane of the surface of bracket 12 andin a direction generally laterally of the U-springs 50 and 52, wherebythe opposite ends of the bight portions 54 of the springs will be offsetlaterally of the springs or, in other words, will assume a generallyskewed relation. On the other hand, as will be apparent from thedrawings, springs, 50 and 52 provide substantially greater stiffness, ifnot substantial rigidity, X-axis, respect to movement of the plate 46 ina direction parallel to the general plane of portion 18 of bracket 12and longitudinally of the springs. Also, as will be apparent from thedrawings, the springs 50 and 52 provide substantially increasedstiffness, if not substantial rigidity, with respect to angular movementof the plate 46 in the general plane of the plate and relative to thebracket 12. The movements of the plate 46 relative to the bracket 12 maybe related to the usual three coordinates of translational motion; forexample, the lateral movement of the plate 46 in the direction of thehorizontal arrow of FIG. 3 may be referred to as motion along theX-axis, while the movement of the plate 46 in the direction toward andaway from bracket 12 may be referred to as mo tion along the Z-axis asshown in FIG. 5. Accordingly, it can also be said that the springs 50and 52 provide substantially increased stiffness to motion of the plate46 in the direction of the vertical arrow of FIG. 3 which may bereferred to as motion along the Y-axis, and also provide substantiallyincreased resistance to angular movement of the plate 46 about the Z-axis.

The intermediate plate 46 is also connected to the top plate 48 by a setof parallel spaced-apart U-springs including a pair of oppositely facingelongated Ushaped U-springs 66 and 68 generally similar incross-sectional configuration to the U- springs 56 and 52 previouslydescribed. The springs 66 and 68 extend along the end edges of theplates 46 and 48, and their ends are respectively rigidly connected tothe plates, by screws 60, nuts 62 and backup strips 64 in the samemanner as described above with respect to springs 50 and 52. As shouldbe apparent from FIG. 5, the arrangement and mounting of the U-springs66 and 68 permit relative movement of the plates toward and away fromeach other in the direction along the Z-axis. Also, the plates 46 and 48are permitted relative movement in a direction parallel to the generalplane of the plates and generally laterally of the U-springs 66 and 68in the direction of the arrow of FIG. 3 along the Y-axis. Also, as willbe apparent, the U-springs 66 and 68 provide substantially increasedstiffness with respect to relative movement of the plates 46 and 48 in adirection generally longitudinally of the springs 66 and 68 or along theX-axis and also provide substantially increased stiffness, if notsubstantial rigidity with respect to angular movement of the plate 48about the Z-axis.

Thus, each suspension element comprises a first set of parallelspaced-apart U-springs and a second set of parallel spacedapartU-springs longitudinally extending in a general plane parallel to thegeneral plane of the first set and arranged at right angles to the firstset. The sets of U-springs are connected in series relation with thefree ends of the sets being respectively connected to the base 11 andmass 13 as shown in FIG. I. From the above, it can be seen that withbracket 12 rigidly mounted, the top plate 48 will be permittedresiliently restrained translational movement in all directions withinthe general plane of the plate and also will be permitted resilientlyrestrained movement toward and away from the bracket 12. On the otherhand, the U-springs 50 and 52 and 66 and 68 will provide substantiallyincreased stiffness with respect to the angular movement of the plate 48relative to the bracket 12 and in the general plane of the plate 48. Interms of the three coordinates of translational movement, the plate 48will be permitted resiliently restrained translational movement alongthe X-, Y- and Z-axes as well as translational movement in anycombination of these axes but will be restrained with a substantiallyincreased stiffness with respect to angular movement of the plate 48about the Z-axis of the suspension element.

The top plate 48 has integrally formed therewith a projection 70 havingan opening 72 passing therethrough and an opening 73 which passespartially therethrough. Projection 70 has a portion 74 which extendsabove the surface of top plate 48 and a portion 76 which extends throughthe opening 32 in bracket 12. Portion 74 with openings 72 and 73 serveas a means on which the mass to be isolated is mounted or affixed toisolator 10. A rubber snubber 78 surrounds portion 76 of projection 70and abuts one surface of plate 48. Snubber 78 cushions the portion 76 ofprojection 78 from any severe shocks to which the isolator 10 may besubjected. Portion 76 and snubber 78 limit movement of plate 48 in thedirection parallel to vertical portion I8 of bracket 12 and also in thedirection toward vertical portion 18. Although the mass is affixed toisolator 10 by means of projection 70, other means, such as, forexample, holes in top plate 48, could be used for affixing the mass tothe isolator.

It may be desirable to utilize the isolators 16 in an iso-elastic systemas shown in FIG. 1. The term iso-elastic" is used herein to define asystem wherein the system will provide equal stiffness with respect totranslational movement of the supported mass along the X-, Y-, andZ-axes of the system as a whole. This system is achieved, in part, inthe embodiment in FIG. 1, by inclining the plurality of suspensionelements upwardly and outwardly of the base II at an acute angle a Inthe system shown, three equally spaced suspension elements 114 areemployed. The angle a as shown in FIG. I, may be defined as the anglebetween the general plane of the top plate 48 of the suspension elementor the general plane of the suspension element and the general planebase 11. The isoelastic characteristic of the system is also achieved,in part, by equiangular spacing of the suspension elements about a lineextending vertically through the center of gravity of the supporteddevice and perpendicular to the general plane of the base 11.

The center of gravity of the supported device is indicated at 88 in FIG.1 and may be laterally offset from the geometric center of the mountingring 15. The spring suspension elements are angularly oriented relativeto the center of gravity 80 of the supported device so that the Z-axesof the suspension elements intersect substantially at a point on thevertical line extending through the center of gravity 80 andperpendicular to the general plane of the base 11. The suspensionelements are offset vertically below the center of gravity 80 of thesupported device a predetermined amount in order to locate the elasticcenter substantially at the center of gravity to meet the firstrequirement of a fully decoupled system as previously stated. It will,of course, be apparent that the suspension elements could be inclined inthe opposite direction so as to extend upwardly and inwardly of the base11 in which case the suspension elements would be offset verticallyabove the center of gravity 80 in order to achieve the desireddecoupling.

In an iso-elastic decoupled system constructed as thus far described,the linear forces acting on the supported device will tend to resultonly in pure translational movement of the device and will be met byequal restraint regardless of the direction of such forces. With respectto rotational input forces, for example, directed angularly about theZ-axis of the system as a whole, the suspension elements will providesubstantially increased restraint or stiffness. The Z-axis of thesystem, in the case of the embodiment of FIG. 1 corresponds to thevertical line 82 passing through the center of gravity 80 and may bereferred to as the azimuth axis of the system. Increased azimuthstiffness of the system is in part derived from the high stiffness ofthe suspension elements with respect to angular displacement of the topplate 48 about the Z-axis of the suspension element, which, of course,is different from the Z-axis of the system as a whole. Morespecifically, the portion of the azimuth restraint which is attributableto the stiffness of the suspension elements about their Z-axes varieswith the cosine squared of the angle a Accordingly, the azimuthstiffness of the system will decrease as the angle a is increased. Alsoin the embodiment of FIG. 1, the stiffness of the system with respect toangular movements of the device about a tilt axis extendingperpendicular to the azimuth axis 82 is substantially increased ascompared to the restraint against translational movement. The restraintagainst angular movement of the supported device about a tilt axisvaries as the sine squared of the angle a and thus restraint increasesas the angle a is increased. Therefore, in the iso-elastic system ofFIG. 1, wherein the suspension elements are inclined at an acute angleto the base 11, the system will afford substantially increased stiffnesswith respect to angular movements about either the azimuth axis or thetilt axis as compared to the restraint offered with respect to purelytranslational movement of the supported device along the X-, Y, Z-axesof the system. It will, of course, also be apparent that with thesuspension elements inclined to provide the iso-elastic feature of thesystem the stiffness with respect to angular movement about he azimuthaxis will not be as great as would be the case where the suspensionelements lie in a general plane extending parallel to the general planeof the ring I so that the Z axes of the suspension elements are parallelto the azimuth axis of the system. With the suspension elements sooriented, the system stiffness about the Z or azimuth axis will be atits maximum, while the system stiffness about a tilt axis will be at itsminimum. Also, as will be apparent, the stiffness of the system withrespect to angular movement about a tilt axis will not be as great aswill be afforded with the suspension elements arranged to lie in a planeextending parallel to the azimuth axis so that the Z-axes of thesuspension elements coincide with radii from the azimuth axis. In thisorientation of the suspension elements azimuth stiffness of the systemwill be at its minimum and tilt stiffness at its maximum. It willfurther be apparent that the inclining of the suspension elements may bedefined by an angle B, which is the angle between the general plane of asuspension element and the azimuth axis 82 of the system, and whichangle B is the complement of the angle a.

In order to provide suspension elements of the requisite stiffness forthe support of the device to be isolated, and which will permitdisplacements of the device which are relatively large with respect tothe stiffness of the suspension elements, and at the same time toprovide a suspension element of a relatively small size, the U-springs50, 52,66 and 68 of the suspension element are provided in a laminatedconfiguration as described in US. Pat. No. 3,066,905. More specifically,each U-spring comprises a plurality of nested U-spring members. Each ofthe U-spring members comprises a bight or web portion which is generallysemicylindrical in cross section. Each U-spring member also includes apair of ends or the like extending from the ends of the bight portionsfor the mounting of the U-spring elements to the plate members aspreviously described. The bight or web portions of the U-spring elementsmay be spaced apart or gapped so as to substantially reduce, if notprevent, rubbing contact between the bight portions during flexurethereof. For example, in a specific embodiment using U-shaped laminates0.004 inch thick, the webs were gapped 0.002 inch at apex of theirapexes. In this manner, interfacial friction within the U-spring issignificantly reduced if not substantially eliminated, and accordingly,any hysteresis effect on the system due to such interfacial friction issubstantially eliminated. Hysteresis effect of an isolation systemrefers to the characteristic of the system whereby followingdisplacement of the supported mass from an initial position, the mass isreturned by the suspension element to a position perhaps slightlydisplaced from its original position. In the case of a navigationalinstrument, it may be particularly desirable to reduce this hysteresiseffect with respect to angular displacement of the device about itsazimuth axis as well as about any tilt axis, such as a roll or pitchaxis.

In some instances, a reduction in hysteresis effect resulting fromfriction within the suspension elements 14 may not be as important asproviding a relatively high degree of damping within the springsuspension elements. In such an instance, the U-springs may beconstructed wherein the laminates, and particularly the deformable bightor web portions thereof, are in nesting contact with each other. Withthis construction there will, of course, be a larger amount ofinterfacial friction between the laminates during deformation of the webportions which interfacial friction may provide a relatively high degreeof damping in the system. While the laminates may be nested in merecontact with each other, also, if desired, each laminate may be providedwith a cross section smaller than that of the next adjacent laminatenested therein. When such laminates are assembled, each outer laminatewill firmly and resiliently embrace and grip the next adjacent innerlaminate, thus increasing the interfacial friction between the laminatesduring deformation of the spring. In this connection, it is preferredthat the radius of the bight portion of each outer laminate be less thanthe radius of the bight portion of the next adjacent inner laminate. Itis also preferred that each outer laminate be dimensioned so that whenit is in its unassembled condition, its legs will be spaced apart adistance less than the spacing of the next adjacent inner laminate. Ascan be seen in FIG. 3, the U- spring is mounted to the respective platesin a manner such that the connecting bight or web portion does not beginimmediately adjacent the edges of the plate but is spaced outwardlytherefrom. However, if desired, the springs, whether incorporatinggapped laminates or not, may be positioned relative to the plate so thatthe bight or web portion begins immediately adjacent the edges of theplate in order to provide a spring characteristic varying from that ofthe case where the bight portion does not begin immediately adjacent theedges of the plate. Thus, it will be apparent that the springcharacteristics of the suspension system may be modified as desired notonly by varying the configuration of the bight or web portion of thespring but also by varying the location of the web portion relative tothe plate connected by the U-spring. Also, it should be apparent thatwhile in the specific embodiment shown, the U-springs are constructedand configured to provide the same stiffness with respect to translationalong the X-, Y-, and Z-axes of the suspension element, if desired, thecharacteristics or configuration of the two sets of U-springs 50 and 52and 66 and 68 may be different to provide different characteristics orstiffness with respect to and between the X-, Y- and Z-axes of thesuspension element.

Where an isolator constructed in accordance with this inventionincorporates U-springs wherein the laminates are gapped, the U-springsthemselves will provide virtually no friction damping. Therefore, inaccordance with this invention, friction dampers 16 are provided asintegral parts of the suspension unit. In the embodiment shown, thesuspension unit is provided with four balanced friction dampers 16. Twodampers 116 are provided to be positioned adjacent opposite sides of topplate 48 as seen in FIG. 1 and the other two dampers 16 are positionedinside the wall of the portion of bracket 12 between openings 28 and 30so as to be adjacent backup strips 64 which are connected to thecenterplate 46 as seen in FIG. 3.

Each of the friction dampers 16 is identical in construction. in thecase of the two dampers adjacent top plate 48 they are housed in twohousings 90 and 92 which are formed integrally with the two backupstrips 64 used to mount springs 50 and 52 to centerplate 46. (See FIGS.1 and 5). In the case of the other two dampers 16, they are positionedinside the wall of the vertical portion 18 of bracket 12 betweenopenings 28 and 30 as seen in FIGS. 3 and 6. Each damper 16 comprises aflat strip of friction material 94 backed by a flat rubber backer 96which in turn is backed by a flat metal plate 98. A threaded insert 100projects from inside the wall beyond the surface of vertical portion 118or from inside housing 90 and 92 beyond the outer surfaces thereof. Athreaded screw 1102 is threadably engageable with insert 100 so thatscrew 102 may be adjusted to bear against plate 98 with eitherincreasing or decreasing pressure. In the case of the dampers l6enclosed in housings 9d and 92, their friction materials 94 bear againstflanges (cannot be seen) which are formed on the sides of top plate 48directly adjacent friction materials 94. In the case of the two dampersl6 enclosed in the wall of vertical portion 18 of bracket 12, theirfriction materials bear against the backup strips 64 which are used infixing springs 66 and 68 to centerplate 46.

Preferably, the friction material 94 is made of a graphite-impregnatedphenolic resin. However, other types of synthetic thermosetting resinsas well as nylon, and certain metals could be employed as the frictionmaterial. However, the greatest efficiency and wear was obtained fromthe phenolic resin. Furthermore metal friction materials have a tendencyto squeak. Whatever friction material is employed, it must have aconstant coefficient of friction so that once screw 102 is adjusted toprovide a particular friction value, that coefficient of friction doesnot change with either time or temperature. Friction damping isimportant because in the absence of friction or so-called Coulomb-typedamping, the transmissibility of the mass goes to infinity at resonanceor the natural frequency. The damping coemcient Q should be selected bysetting a particular value of friction damping so as to maintain thetransmissibility of the mass at a specified level at resonance. Once thefriction damping is set for a given system, it is desired to maintainconstant damping so that the selected Q setting (usually, for example,between one and 10) does not change. The metal plate 98 permits uniformpressure to be applied by screw 102 all across the surface of the plate.Rubber backer 9b maintains a constant force on the friction material 94and compensates for any wear of the friction material.

The surface shape of the flat friction material 94 and backer 96, andplate 98 may be selected from any of a number of configurations. Thesurface shape of each may be rectangular, square, circular or any otherdesired configuration. The area of the surface of each of these membershas no effect on the friction provided but only effects how fast thematerial wears. It has been found that the rectangular shape ispreferable because it wears the longest.

As previously mentioned, the number and positioning of 70 frictiondampers 16 is such as to provide balanced friction damping in that theapplication of motion in any direction generates friction forces ormoments only in the direction of applied motion and not in the directionof the other five coordinates of motion. None of the dampers 116 shouldbe focused at the elastic center of the system so that friction dampingis provided in the three coordinates of translational motion and also inthe three coordinates of rotational motion. In the embodiment shown, thefriction dampers l6 housed in housings 9t) and 92 provide frictiondamping in the direction of the X- and Z-artes with respect totranslational motion. The dampers 16, carried in the wall of verticalportion I8 of bracket I2, provide friction damping in the direction ofthe Y-axis with respect to translational motion.

Although the embodiment of the present invention as described and shownin the figures has freedom of movement in two translational degrees offreedom, it is also possible to incorporated the present invention in anisolator having only tow degrees of freedom. As viewed from FIG. 3 suchan embodiment of the present invention might have translational degreesof freedom in the directions of the X- and Y-axes but not in thedirection of the Z-axis. Another embodiment of the invention having twodegrees of freedom might have translational degrees of freedom in thedirections of the X- and Z- axes but not in the direction of the Y-axis.Still another version could have degrees of freedom in the direction ofthe X- and Z-axes but not in the direction of the X-axis.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted in an illustrative andnot in a limiting sense.

What is claimed is:

l. A vibration isolator suspension element for use in a vibrationisolating system comprising:

a first set of parallel spaced-apart U-springs;

a second set of parallel, spaced-apart U-springs longitudinallyextending at right angles to the first set of U- springs and in ageneral plane extending parallel to the general plane of the first setof U-springs;

means for connecting said first and second set of U-springs in seriesrelation;

means for mounting one end of said suspension element on a supportingstructure;

means for mounting on the other end of said suspension element a mass tobe supported; and

means provided integrally with said suspension element for providingfriction damping during both translational and rotational motion towhich said supported mass may be subjected.

2. An element as set forth in claim 1 wherein said element is employedin a fully decoupled system.

3. An element as set forth in claim 1 wherein said friction dampingmeans is positioned so as to provide balanced friction damping.

4. An element as set forth in claim 1 wherein said friction dampingmeans are positioned so that the lines of action of said damping meansare not focused at the elastic center of said isolating system.

5. An element as set forth in claim 1 wherein said friction clampingmeans comprises a plurality of individual friction dampers positioned onan integral with said suspension element so as to provide balancedfriction damping, each of said dampers including means for applying afriction force to said means for connecting said first and second setsof U-springs; and

means for adjusting the amount of friction force applied by each of saidfriction-force-applying means.

6. An element as set forth in claim 5 wherein said means for applying afriction force comprises a relatively flat member made of a materialhaving a substantially constant coefficient of friction.

7. An element as set forth in claim 6 wherein said material is agraphite-impregnated phenolic resin.

8. An element as set forth in claim 1 wherein said means for connectingsaid first and second set of U-springs includes adjustable means foraccurately setting and maintaining the spring rate of each individualU-spring.

9. A vibration isolator suspension element for use in a vibrationisolating system, said element comprising:

a first set of parallel, spaced-apart U-springs;

a second set of parallel, spaced-apart U-springs longitudinallyextending at right angles to said first set and lying in a planeextending parallel to the general plane of said first set of U-springs;

means for connecting said first and second set of U-springs in seriesrelation so that the opposite ends of said suspension elementcorresponding to the opposite ends of the series connected sets ofU-springs are relatively movable in two directions extending at rightangles and lying in a common plane extending parallel to the generalplane of said suspension element and are relatively movable in a thirddirection extending at right angles to the general plane of saidsuspension element;

mean for mounting one end of said suspension element on a supportingstructure with one end of each series connected sets of U-springsconnected to the supporting structure;

means for connecting the other end of each series connected sets ofU-springs to a mass to be carried by the isolating system; each of saidU-springs including a plurality of U-spring members each having aresiliently deformable web portion. The U-spring members of eachU-spring being in nested relation with at least the web portions thereofbeing in spaced relation; and

means provided integrally with said suspension element for providingfriction damping during both translational and rotational motion towhich said mass may be subjected;

said friction damping means comprising a plurality of individualfriction dampers positioned on and integral with said suspension elementso that the lines of action of said dampers are not focused at theelastic center of said isolating system and so as to provide balancedfriction damping.

10. An element as set forth in claim 9 wherein each of said dampersincludes means for applying a friction force to said means forconnecting said first and second sets of U-springs, said friction meanscomprising a relatively fiat member made of a material having asubstantially constant coefficient for friction; and

means for adjusting the amount of friction force applied by each of saidfriction-force-applying means.

11. An element as set forth in claim 9 wherein said means for connectingsaid first and second set of U-springs includes adjustable means foraccurately setting and maintaining the spring rate of each individualU-spring.

12. A vibration isolating system having an azimuth axis and comprising:

a supporting structure;

a plurality of suspension elements spaced angularly about said azimuthaxis and each comprising a first set of parallel, spaced-apart U-springsto second set of parallel, spaced-apart U-springs, longitudinallyextending at right angles to the first set of U-springs and in a generalplane extending parallel to the general plane of the first set ofU-springs;

means connecting said first and second set of U-springs of eachsuspension element in series relation so that the opposite ends of eachsuspension element corresponding to the opposite ends of the seriesconnected sets of U- springs are relatively movable in two directionsextending at right angles and lying in a common plane extending parallelto the general plane of the suspension elements and are relativelymovable in a third direction extending at right angles to the generalplane of the suspension element;

means for mounting one end of each suspension element on said supportingstructure;

means for mounting on the other end of each of the suspension elements amass to be carried by the isolating s stem; the general plane of eachsuspension element exten ing at

1. A vibration isolator suspension element for use in a vibrationisolating system comprising: a first set of parallel spaced-apartU-springs; a second set of parallel, spaced-apart U-springslongitudinally extending at right angles to the first set of U-springsand in a general plane extending parallel to the general plane of thefirst set of U-springs; means for connecting said first and second setof U-springs in series relation; means for mounting one end of saidsuspension element on a supporting structure; means for mounting on theother end of said suspension element a mass to be supported; and meansprovided integrally with said suspension element for providing frictiondamping during both translational and rotational motion to which saidsupported mass may be subjected.
 2. An element as set forth in claim 1wherein said element is employed in a fully decoupled system.
 3. Anelement as set forth in claim 1 wherein said friction damping means ispositioned so as to provide balanced friction damping.
 4. An element asset forth in claim 1 wherein said friction damping means are positionedso that the lines of action of said damping means are not focused at theelastic center of said isolating system.
 5. An element as set forth inclaim 1 wherein said friction damping means comprises a plurality ofindividual friction dampers positioned on an integral with saidsuspension element so as to provide balanced friction damping, each ofsaid dampers including means for applying a friction force to said meansfor connecting said first and second sets of U-springs; and means forAdjusting the amount of friction force applied by each of saidfriction-force-applying means.
 6. An element as set forth in claim 5wherein said means for applying a friction force comprises a relativelyflat member made of a material having a substantially constantcoefficient of friction.
 7. An element as set forth in claim 6 whereinsaid material is a graphite-impregnated phenolic resin.
 8. An element asset forth in claim 1 wherein said means for connecting said first andsecond set of U-springs includes adjustable means for accurately settingand maintaining the spring rate of each individual U-spring.
 9. Avibration isolator suspension element for use in a vibration isolatingsystem, said element comprising: a first set of parallel, spaced-apartU-springs; a second set of parallel, spaced-apart U-springslongitudinally extending at right angles to said first set and lying ina plane extending parallel to the general plane of said first set ofU-springs; means for connecting said first and second set of U-springsin series relation so that the opposite ends of said suspension elementcorresponding to the opposite ends of the series connected sets ofU-springs are relatively movable in two directions extending at rightangles and lying in a common plane extending parallel to the generalplane of said suspension element and are relatively movable in a thirddirection extending at right angles to the general plane of saidsuspension element; mean for mounting one end of said suspension elementon a supporting structure with one end of each series connected sets ofU-springs connected to the supporting structure; means for connectingthe other end of each series connected sets of U-springs to a mass to becarried by the isolating system; each of said U-springs including aplurality of U-spring members each having a resiliently deformable webportion. The U-spring members of each U-spring being in nested relationwith at least the web portions thereof being in spaced relation; andmeans provided integrally with said suspension element for providingfriction damping during both translational and rotational motion towhich said mass may be subjected; said friction damping means comprisinga plurality of individual friction dampers positioned on and integralwith said suspension element so that the lines of action of said dampersare not focused at the elastic center of said isolating system and so asto provide balanced friction damping.
 10. An element as set forth inclaim 9 wherein each of said dampers includes means for applying afriction force to said means for connecting said first and second setsof U-springs, said friction means comprising a relatively flat membermade of a material having a substantially constant coefficient forfriction; and means for adjusting the amount of friction force appliedby each of said friction-force-applying means.
 11. An element as setforth in claim 9 wherein said means for connecting said first and secondset of U-springs includes adjustable means for accurately setting andmaintaining the spring rate of each individual U-spring.
 12. A vibrationisolating system having an azimuth axis and comprising: a supportingstructure; a plurality of suspension elements spaced angularly aboutsaid azimuth axis and each comprising a first set of parallel,spaced-apart U-springs to second set of parallel, spaced-apartU-springs, longitudinally extending at right angles to the first set ofU-springs and in a general plane extending parallel to the general planeof the first set of U-springs; means connecting said first and secondset of U-springs of each suspension element in series relation so thatthe opposite ends of each suspension element corresponding to theopposite ends of the series connected sets of U-springs are relativelymovable in two directions extending at right angles and lying in acommon plane extending parAllel to the general plane of the suspensionelements and are relatively movable in a third direction extending atright angles to the general plane of the suspension element; means formounting one end of each suspension element on said supportingstructure; means for mounting on the other end of each of the suspensionelements a mass to be carried by the isolating system; the general planeof each suspension element extending at an acute angle to said azimuthaxis, the U-springs of the suspension elements providing the soleresilient supporting force for a mass carried by the isolating system;and means provided integrally with each of said suspension elements forproviding balanced friction damping during both translational androtational motion to which said mass may be subjected.